X-ray mammography has long been considered the "gold standard" of breast imaging and is well-developed and capable of yielding outstanding image resolution. In x-ray mammography, x-rays are transmitted through the patient's breast and impinge upon x-ray film or a digital imaging camera. Internal features of the patient's breast are depicted as shadows in the resulting image due to differences in x-ray absorption between differing types of tissue. Ordinarily, the patient's breast is compressed during x-ray mammography so that the breast is of more uniform thickness, thereby enhancing the resulting image. In addition to pure imaging applications such as patient screening or diagnosis, x-ray mammography has been used to localize breast lesions for needle biopsy or for placement of a wire or markings to guide a surgeon during subsequent surgical biopsy.
Recently, magnetic resonance imaging (MRI) has been investigated for use in breast imaging. Generally, in MRI, atomic nuclei which possess nuclear magnetic momentum, or spin, are first aligned in a relatively strong magnetic field and are then excited by radio frequency (RF) resonance energy. When their spin returns to equilibrium they emit energy in the form of an RF signal. The time of emission of the absorbed energy, called the relaxation time, depends on magnetic properties of the tissue. The emitted signal can be analyzed to obtain imaging information regarding the tissue under examination. Ordinarily, specified regions or slices of the tissue are sequentially examined so that a composite image of the tissue under examination is obtained.
MRI is of interest to breast imaging specialists for various reasons. Initially, MRI avoids cumulative radiation exposure, a concern expressed by some in connection with alternative x-ray imaging techniques. Additionally, MRI reduces the need to compress the patient's breast to a uniform thickness, thereby potentially enhancing patient comfort. Moreover, MRI may provide imaging advantages for certain soft tissue imaging applications due to its reliance on nuclear magnetic relaxation times rather than x-ray absorption or other characteristics. Advances in MRI technology are also improving MRI resolution.
However, as to breast imaging, a number of challenges remain in realizing the full potential of MRI. For example, conventional tunnel MRI devices where a patient is axially positioned within a magnetic tunnel are intended for obtaining a composite image from axial body slices. These systems are not tailored for breast imaging and, standing alone, are not capable of yielding the images necessary for increasingly acute breast diagnostic techniques.
Moreover, known MRI devices are not adapted for use in localizing a breast lesion so as to permit insertion of a medical instrument to sample, excise or treat the lesion. With regard to conventional tunnel MRI devices, due to space limitations within the tunnel bore, it is impractical to perform procedures involving insertion of a medical instrument into a patient's breast while the patient remains in an imaging position within the bore. In addition, such devices do not provide a mechanism for allowing localization of a breast lesion after the patient has been withdrawn from the tunnel bore. Known MRI devices which are dedicated to breast imaging generally include magnetic generators and signal transmitters/receivers which surround the patient's breast. These components can interfere with conventional medical instrument insertion devices.